Preheat oven to 325 degrees F. Grease and flour a 10-inch-by-5-inch or 9-inch-by-5-inch loaf pan.
In a large bowl, mash bananas with a fork or potato masher.
Add melted butter, brown sugar, granulated sugar, eggs, vanilla, and buttermilk. Whisk until thoroughly combined.
Add dry ingredients to the bowl with the banana mixture. Using a rubber spatula or wooden spoon, stir until flour is no longer visible. Fold in the mini chocolate chips.
Transfer batter to the prepared loaf pan.
Bake for 70-80 minutes, or until a toothpick inserted in the center comes out mostly clean. Start checking your bread at the 70-minute mark every 5 minutes to prevent over baking.
Allow the bread to cool completely in the pan. Run a butter knife around the edges of the pan to loosen the loaf. Remove from the pan and allow to cool completely. Frost with the mocha buttercream. Store in an airtight container for up to 3 days or slice and freeze for up to 3 months.
To make the mocha buttercream, add butter and powdered sugar to the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl using a handheld mixer) and beat until smooth. Add the cocoa powder, espresso powder, vanilla extract, and coffee extract. Beat until combined. Scrape down the sides of the bowl. With the mixer on low, slowly add the heavy cream. Gradually turn the speed up to high and beat for 30 seconds.